There is a way you can check if your answer makes sense. Since you have found the object distance, 9.37cm, you can get the image distance, which is -3.59*9.37 = 33.64 cm (from your magnification equation). If you put both of those back into your lens equation so (1/f) = (1/9.37)+(-1/33.64) it should work out to your focal length that you were given in the question. If it doesn't, you have made a mistake somewhere. Try it. It's a useful way to check your work on tests.